Technical Field
The present disclosure relates to a non-destructive inspection device for observing an internal structure of a subject with high sensitivity utilizing properties as waves of radioactive rays transmitted through the subject, e.g., X-rays.
Description of the Related Art
Radioactive rays having high transmission power such as X-rays are widely used as a probe for seeing through an object's interior in medical image diagnosis, non-destructive inspection, security check and the like. Contrast of an X-ray radiograph differs depending on an X-ray attenuation ratio, and an object which strongly absorbs X-rays is drawn as the shadow of the X-rays. X-ray absorption capability becomes stronger as more elements with large atomic numbers are contained. Conversely, it can be pointed out that an object composed of elements with small atomic numbers has low contrast. This is a principle defect of X-ray radiographs. Thus, sufficient sensitivity cannot be obtained for biological soft tissues and polymers.
On the other hand, if the properties as waves of X-rays are utilized, higher sensitivity potentially about three orders of magnitude larger than general conventional X-ray radiographs can be realized. Hereinafter, this is called an X-ray phase contrast method. If this technique is applied to the observation of an object (biological soft tissue, polymers or the like) composed of light elements which do not absorb X-rays very much, an inspection which has been difficult by conventional methods is enabled. Thus, its practical use is expected.
To realize a high-sensitivity imaging method utilizing the X-ray phase contrast method, X-ray optical systems using monochromatic plane waves of X-rays have been mainly studied so far, wherefore the use of an X-ray source having very high luminance is assumed.
To obtain monochromatic plane waves, only specific spectrum components which propagate in a specific direction need to be selected from originally obtained X-rays. Thus, to ensure intensity necessary for imaging, the original X-rays are required to have brightness sufficient to compensate for loss caused due to the selection. A single crystal such as silicon is used as an optical element for performing such a selection, but the use of a huge synchrotron radiation facility has to be substantially assumed, which is a large obstacle in considering practical use.
If the X-ray phase contrast method that functions with a cone beam having a wide band width is realized, a device using a compact X-ray source other than synchrotron radiation can be expected. An X-ray phase contrast method by an X-ray Talbot interferometer is expected as a candidate of such an imaging method (see patent literature 1 and 2 below). Since not single crystals, but X-ray gratings are used in this method, imaging utilizing polychromatic diverging X-rays is possible.
However, in the phase contrast method, X-rays are required to have a certain degree of spatial coherence. For that, the size of an X-ray generation source has to be reduced to a certain extent. Then, the pre-existing compact X-ray sources usable in this method are substantially micro-focus X-ray sources. Normal-focus X-ray sources are not applicable to this.
In the micro-focus X-ray source, X-rays are generated by irradiating electron beams to a minute area of a target. Many electrons need to be irradiated when it is desired to generate many X-rays. However, due to a problem of thermal load in the target, an upper limit of actual X-ray power is restricted. As a result, if X-ray imaging is performed, assuming the power of X-rays obtained by the micro-focus X-ray source, there is a problem of extending an exposure time.
In a conventional X-ray Tablot-Lau interferometer, a problem of intensity shortage is avoided using a normal-focus X-ray source. The X-ray Tablot-Lau interferometer is so configured that a multi-slit is added between an X-ray source and a G1 grating of an X-ray Talbot interferometer (see patent literature 3, 4 and non-patent literature 1 below). It should be noted that the multi-slit is called a G0 grating in some cases, but this multi-slit is for configuring virtual X-ray sources. Specifically, in this technology, X-rays are generated by irradiating electron beams to a relatively wide area on the target and the generated X-rays are partly transmitted through the multi-slit. In this way, it is possible to realize an X-ray source in which narrow and linear virtual X-ray sources are arranged at predetermined pitches. It should be noted that an X-ray source having a plurality of microlines is called a micro multi-line X-ray source in some cases.
The X-ray Tablot-Lau interferometer can be called a technology realizing the X-ray source in the X-ray Tablot interferometer by the normal-focus X-ray generator and the multi-slit and should be called a specific form of the X-ray Tablot interferometer.
In a phase contrast method by X-ray Tablot interferometers including Tablot-Lau interferometers, a moiré pattern image is recorded by an X-ray detector. In this image, an effect of X-ray refraction by a subject is visualized. However, absorption contrast corresponding to conventional images is also superimposed and contrast unrelated to the subject is also added due to the imperfection of an imaging optical system and a device. To separate these and conduct a more precise and advanced inspection, development towards a quantitative image measurement technology called an “X-ray phase imaging method” has been proposed. To realize the phase imaging method in the X-ray Tablot interferometer, procedures called a “fringe scanning method” (non-patent literature 2) and a “Fourier transform method” (non-patent literature 3 and non-patent literature 4) are carried out. The fringe scanning method is a method for obtaining an absorption image, a refraction image and a scattering image through computer arithmetic processing by moving any one of gratings by a predetermined step amount each time and successively imaging a subject to obtain a change of a moiré pattern in the form of a plurality of pieces of image data. On the other hand, the Fourier transform method is a method for similarly obtaining an absorption image, a refraction image and a scattering image from one moiré fringe image through predetermined Fourier filtering by inclining one grating to generate fine rotational moiré fringes. The Fourier transform method has a problem of being inferior in spatial resolution as compared to the case where the fringe scanning method is applied. Thus, the fringe scanning method is considered to be suitable for detailed inspection of the subject. It should be noted that the absorption image corresponds to a conventional image, the refraction image is an image mapping deflection angles of X-rays by refraction in the subject and the scattering image is an image mapping a reduction in the visibility of the moiré pattern by the subject. Since this visibility reduction corresponds to a distribution of fine scatterers included in the subject, this is called the scattering image.
In the fringe scanning method, the subject stationary in a field of view is imaged while the gratings are successively moved by the predetermined amount. Thus, imaging itself takes several tens to several hundreds of seconds at present.
On the other hand, subjects are successively conveyed by moving means such as a belt conveyor, for example, in a factory. When it is necessary to inspect all the subjects, the imaging of the subjects has to be completed in a short time and defects have to be detected at a high speed. A similar high-speed processing is necessary also in inspecting baggage at airports.
However, in the conventional fringe scanning method, a subject standing still at a predetermined position needs to be imaged a plurality of number of times using a two-dimensional X-ray image detector while shifting the grating by a fraction of an integer of a period of the grating. Thus, there has been a problem of being difficult to inspect subjects moving at a certain speed or higher.
Further, a mechanism for accurately shifting the grating by a very small amount requires considerable mechanical precision, which has presented a problem of imposing a burden in terms of cost and maintenance.